Lightly grease and base-line a 20 cm (8 inch) loose-bottomed round cake tin with nonstick baking paper. Arrange the clementine slices over the base and spoon over the marmalade.
Place the eggs and sugar in a bowl and beat with a hand-held electric whisk until pale and thick and the whisk leaves a trail when lifted above the mixture. Gently stir in the clementine rind and juice and fold in the flour.
Pour over the clementines and place in a preheated oven, 180ºC (350ºF), Gas Mark 4, for 20–25 minutes or until golden brown and firm to the touch. Leave to cool in the tin for a few minutes.
Invert the tin on to a plate and serve the cake in slices with dollops of fromage frais, if using.